Rheumatoid arthritis causes significant disability and increased mortality. Research into its pathogenesis currently focuses on the mechanisms by which host immunity becomes dysregulated to produce pathologic changes. Macrophage migration inhibitory factor (MIF) is an upstream regulator of the immune response that is produced both by innate (monocyte/macrophage) and adaptive (T cell) immune cells. We have discovered that the human MIF gene (Mif) is encoded by functionally distinct alleles, and that high-expression Mif alleles are over-represented in patients with rheumatoid arthritis. We also have identified the cell surface protein, CD74, to be a cellular receptor for MIF. We suggest that MIF overproduction by the innate or the adaptive immune response and its binding to CD74 play an intrinsic role in the pathogenesis of rheumatoid arthritis. The objective of this application is to define the cellular basis of MIF production and action in inflammatory arthritis. The central hypothesis is that inflammatory arthritis results from an overproduction of MIF by monocytes/macrophages or by T cells, coupled with MIF activation of target cells by binding to CD74. The rationale for this proposed research is that once the molecular basis for MIF production and action in inflammatory arthritis is known, then a more precise understanding of immune dysregulation in this disease will have been achieved. We will test our hypothesis and accomplish the objective of this application by pursuing three specific aims: 1) Define MIF's Role in Inflammatory Arthritis by Studying Monocyte-specific and T cell-specific Mif-Knockout Mice. We will examine arthritis development in Mif-floxed mice made genetically-deficient either for monocyte-derived MIF, or for T cell-derived MIF. 2) Establish the Pathologic Role of the MIF/CD74 Interaction In Vivo. We will establish the pathologic importance of the MIF/CD74 interaction by administering a neutralizing anti- CD74 mAb, or a soluble CD74 protein (sCD74), to wild-type mice with experimental arthritis. 3) Define the Role of High- versus Low-expression Mif alleles in MIF Production, Cellular Activation, and Downstream Cytokine Expression by Human Monocytes and T Cells. We will obtain monocytes and T cells from patients with rheumatoid arthritis, analyze the Mif genotype, and study MIF production, cellular activation, and downstream cytokine expression. These results will be significant because they will provide a more precise understanding of MIF's role in the pathways that link innate and adaptive immunity, and which become dysregulated to produce inflammatory arthritis. Moreover, they will provide a scientific rationale for identifying individuals who may be a high risk for severe rheumatoid arthritis - based on their Mif alleles, so that steps can be taken to better treat their erosive disease.